The present invention relates to magnetic memory devices, and more particularly to systems for detecting servo data between tracks in magnetic disks.
Magnetic systems, such as disk drives, are used to store large amounts of user data. In disk drives, the user data is written onto concentric tracks or disk-shaped magnetic media. Each track is defined by a circular path on the disk. Commonly, beyond the user data, additional positioning information, known as servo data, is written periodically between tracks. Both user data and servo data are magnetic markings written on the media. The servo data are used to guide the read/write heads, which are normally used to read and write user data, to a proper position. The frequency characteristics of user data and servo data are quite different. While the spectrum of user data occupies a fairly wide band, depending on the specific information written, the spectrum of the servo data is quite narrow, and typically somewhat lower than the highest frequencies that are used for user data. One of the reasons for the lower frequency is to provide good noise immunity, which is particularly important in reading servo data, in order to guarantee proper positioning of the read/write head and thereby reduce read-error rate. Better noise immunity is achieved by setting the frequency spectrum of the servo data to be in the region of the spectrum where there is no significant attenuation due to magnetic head and media effects.
It is important to optimize this noise immunity in order to increase head positioning error and reduce the read-error rate of the device. Therefore, disk drive read channel manufacturers try to optimize noise characteristics of their devices differently for reading user data as opposed to servo data. As an example, FIG. 1 shows a section 5 of a typical prior art read channel utilizing the so-called PRML (Partial-Response Maximum-Likelihood) data-detection method. The PRML method improves the data throughput and increases areal density, compared to earlier data-detection methods used in disk drive magnetic recording.
As shown in FIG. 1, an analog input signal 10, which is the amplified read output signal of a magnetic head, is passed through a programmable low pass filter (LPF) 20 to attenuate high frequency noise. After the LPF 20, the signal is digitized by analog to digital (A/D) converter (ADC) 30, where it is digitized to accommodate further processing in the digital domain. After the signal has been digitized, it is passed through digital finite impulse response (FIR) filter 40, which provides frequency response equalization to compensate for the head and media frequency response imperfections. The FIR 40 also provides additional low-pass filtering. It should be noted that sometimes FIR 40 is placed prior to ADC 30 to achieve the same equalization and filtering effect in the analog domain.
As the read channel is used to process both user data and servo data, and it is desirable to optimize its characteristics for both modes of operation, a binary servo gate signal 70 is used to change the filtering parameters of the LPF 20 and FIR 40 filters, and to selectively activate the servo demodulator 60. When the servo gate signal 70 is low, the read channel is operated in the user data mode, and when the servo gate signal 70 is high, the read channel is operated in the servo mode. In the user data mode, the output of the ADC 30/FIR 40 combination is processed by the activated data demodulator 50, which produces user NRZ data. In the servo mode, the FIR output is processed by the servo demodulator 60, which produces a servo control signal used by the positioning circuitry (not shown).
Disregarding the head and media response imperfections, FIGS. 2A-2C illustrate the frequency response of the read channel in the user data mode and the effect of the filters on the signal and noise spectra. The user data spectrum 80 and noise spectrum 90, shown in FIG. 2A, on the input to the read channel, is modified by the low-pass filter combination signal 100, shown in FIG. 2B, of the LPF 20 and FIR 40 to produce the spectra 110 and 120, shown in FIG. 2C, for user data and noise respectively. While the overall signal-to-noise ratio has been improved, the in-band noise has not been attenuated.
FIGS. 3A-3C illustrate the frequency response of the read channel of FIG. 1 in the servo mode, and the effect of the filters on the signal and noise spectra. The servo data spectrum 130 and noise spectrum 90 (which is essentially unchanged compared to the user data case), shown in FIG. 3A, received at the input of the read channel, are modified by the low-pass filter combination 140, shown in FIG. 3B, of the LPF 20 and FIR 40 to look like the spectra 150 and 160 for servo data and noise respectively, shown in FIG. 3C. As the highest components of the spectrum of servo data 150 are substantially lower in frequency than the highest components of the user data spectrum 80, the filter response 140 is modified by the servo gate signal 70, compared to the filter response 100 of FIG. 2B, to have a lower cutoff frequency and, thus, noise is reduced more than in the user data case of FIG. 2C, as reflected in noise spectrum 160. However, because the low-pass topology is still used, there are substantial noise components at both high and (especially) low ends of the spectrum which substantially degrade the theoretically possible signal to noise ratio for the servo data.
It is therefore desirable to improve the signal-to-noise ratio of the processed servo signal. It is further desirable to do so for a range of servo signal frequencies which are optimum for a particular head and media combination used in a magnetic recording device.
This invention provides the improvement of signal-to-noise ratio of servo signal detection in read channels. To optimize the signal to noise ratio, the input servo signal is multiplied with a reference signal and then band-pass filtered. In order to address the optimal selection of servo data spectrum based on media characteristics, the placement of the band-pass filter center frequency is made variable. This is achieved by introducing a heterodyne in the path of the analog servo signal, thereby shifting the center of the spectrum and subsequently passing the servo signal through a fixed band-pass filter.
In the preferred form, an analog user data input signal is passed to a low pass filter and then to an A/D converter, to create a digital representation of the user data signal. The digital user signal is then processed by a finite impulse response filter, which provides frequency response equalization to compensate for head and media frequency response imperfections and then it is processed by a data demodulator. The servo data input signal is passed to an oscillator and then to a band pass filter. The servo signal is then passed to an envelope detector and then to an A/D converter, where it is represented as a digital servo signal. From the servo demodulator, the digital signal is then passed to a servo control system.